Hexamethylbenzene

The complete IUPAC name for this compound is 1,2,3,4,5,6-hexamethylbenzene. The locants (the numbers in front of the name) are superfluous, however, as the name hexamethylbenzene uniquely identifies a single substance and thus is the formal IUPAC name for the compound. It is an aromatic compound, with six π electrons (satisfying Hückel's rule) delocalised over a cyclic planar system; each of the six ring carbon atoms is sp2 hybridised and displays trigonal planar geometry, while each methyl carbon is tetrahedral with sp3 hybridisation, consistent with the empirical description of its structure. Hexamethylbenzene is sometimes called mellitene,) Mellite is composed of a hydrated aluminium salt of benzenehexacarboxylic acid (mellitic acid), with formula . Mellitic acid itself can be derived from the mineral, and subsequent reduction yields mellitene. Conversely, mellitene can be oxidised to form mellitic acid: Treatment of hexamethylbenzene with a superelectrophilic mixture of methyl chloride and aluminum trichloride (a source of Meδ⊕Cl---δ⊖AlCl3) gives heptamethylbenzenium cation, one of the first carbocations to be directly observed. Structure In 1927 Kathleen Lonsdale determined the solid structure of hexamethylbenzene from crystals provided by Christopher Kelk Ingold. and was subsequently described as "remarkable ... for that early date". Lonsdale described the work in her book Crystals and X-Rays, explaining that she recognised that, though the unit cell was triclinic, the diffraction pattern had pseudo-hexagonal symmetry that allowed the structural possibilities to be restricted sufficiently for a trial-and-error approach to produce a model. providing crucial evidence in understanding the nature of aromaticity. ==Preparation==

The compound can be prepared by reacting phenol with methanol at elevated temperatures over a suitable solid catalyst such as alumina. what has been described as an "extraordinary" zinc chloride-catalysed one-pot synthesis of hexamethylbenzene from methanol. :15    →     +   3    +   15  Le Bel and Greene rationalised the process as involving aromatisation by condensation of methylene units, formed by dehydration of methanol molecules, followed by complete Friedel–Crafts methylation of the resulting benzene ring with chloromethane generated in situ. Hexamethylbenzene is also produced as a minor product in the Friedel–Crafts alkylation synthesis of durene from p-xylene, and can be produced by alkylation in good yield from durene or pentamethylbenzene. Hexamethylbenzene is typically prepared in the gas phase at elevated temperatures over solid catalysts. An early approach to preparing hexamethylbenzene involved reacting a mixture of acetone and methanol vapours over an alumina catalyst at 400 °C. Combining phenols with methanol over alumina in a dry carbon dioxide atmosphere at 410–440 °C also produces hexamethylbenzene, though as part of a complex mixture of anisole (methoxybenzene), cresols (methylphenols), and other methylated phenols. An Organic Syntheses preparation, using methanol and phenol with an alumina catalyst at 530 °C, gives approximately a 66% yield, though synthesis under different conditions has also been reported. The mechanisms of such surface-mediated reactions have been investigated, with an eye to achieving greater control over the outcome of the reaction, especially in search of selective and controlled ortho-methylation. Both anisole Trimerisation of three 2-butyne (dimethylacetylene) molecules yields hexamethylbenzene. The reaction is catalyzed by triphenylchromium tri-tetrahydrofuranate or by a complex of triisobutylaluminium and titanium tetrachloride. ==Applications and reactions==

Hexamethylbenzene has no commercial or widespread uses. It is exclusively of interest for chemical research. Most applications of hexamethylbenzene are as a chemical feedstock, although it has also been used as a solvent for 3He-NMR spectroscopy. Oxidation with trifluoroperacetic acid or hydrogen peroxide gives 2,3,4,5,6,6-hexamethyl-2,4-cyclohexadienone: Such complexes have been reported for a variety of metal centres, including cobalt, rhodium, Organometallic chemistry The electron-donating nature of the methyl groups—both that there are six of them individually and that there are six meta pairs among them—enhance the basicity of the central ring by six to seven orders of magnitude relative to benzene. Known cations of sandwich complexes of cobalt and rhodium with hexamethylbenzene take the form (M = Co, Fe, Rh, Ru; n = 1, 2), where the metal centre is bound by the π electrons of the two arene moieties, and can easily be synthesised from appropriate metal salts by ligand exchange, for example: :3    +   Al   →   3    +   In the field of organoruthenium chemistry, the redox interconversion of the analogous two-electron reduction of the dication and its neutral product occurs at −1.02 V in acetonitrile The hapticity of one of the hexamethylbenzene ligands changes with the oxidation state of the ruthenium centre, the dication [Ru(η6-C6(CH3)6)2]2+ being reduced to [Ru(η4-C6(CH3)6)(η6-C6(CH3)6)], suggesting a change in structure different from that found in the ruthenium system. Dication As discovered in the 1960s and '70s, two-electron oxidation of hexamethyl Dewar benzene gives pyramid-shaped ions with composition and : : with composition |alt=Gray pentagonal-pyramid skeleton Synthesis from hexamethylbenzene would offer a cheaper feedstock to the same end. Spectroscopic investigation of the two-electron oxidation of benzene at very low temperatures (below 4 K) shows that a hexagonal dication forms and then rapidly rearranges into the same pyramidal structure: Two-electron oxidation of hexamethylbenzene could therefore result in a near-identical rearrangement to a pyramidal carbocation. However, this method has not successfully produced the dication in bulk. == References ==